The META tags are used to provide extra information about a web page. There was once a time where a good search engine ranking could be achieved by simply changing the value of the META tags. However these days are long gone. Nowadays the META Tags are decreasing in importance in the eyes of the search engines who are using more sophisticated methods of ranking pages (namely link popularity, optimized anchor text).
There are many different types of META tags but the one that can affect ranking are the Keywords META tag and the Description META tag.
This tag contains keywords relevant to a page. In the past search engines referenced this to determine how to rank a particular page. However many people started to abuse this and stuffed the tag with keywords that were not relevant to the content of the page. This confused search engines and subsequently returned weaker search results. To combat this less and less emphasis has been placed on the Keywords META tag. However this does not mean that it is entirely useless. Here are some reasons to still include it:
Although most search engines do not use the tag there are undoubtedly still some that do. For this reason place around 15-20 keywords relevant to your site or page into the tag. Do not attempt to cheat the search engines by repeating keywords as this is considered spamming and they may look unfavourably on this.
People can misspell certain keyphrases that you may be optimizing for so it is common to place these misspellings into the Keywords META tag. This allows you to rank in search engine results for misspellings without having to represent them on the actual content of your page.
In the past this tag was used to specify the text that would appear in search engine results alongside the link to your site. Keywords could be placed into this text to influence ranking. However it was abused similar to the Keywords META tag and has befallen the same fate, meaning it is not as useful as it once was. Some search engines do not display it in their results (most notably Google, which retrieves the text from content on your page) and do not use it to rank a site. Having said that some search engines do still use it so again it is a mistake to ignore it completely.
Descriptions placed into this tag should, strangely enough, be very descriptive of the page or site they reside in. Include a few relevant keywords/keyphrases that you are optimizing for in the description and try to limit its length to 25-30 words. Also try to use no more than two sentences.
Although not as important as they once where, the META tags can still influence a search engine when it comes to deciding whether your web page is relevant or not.
15-20 keywords, include misspellings of keywords/keyphrases
Include keywords/keyphrases, 25-30 words, no more than two sentences.